1.Field of the Invention
The present invention relates to a process for cleaning a lubricated mechanical refrigeration system. In particular, the present invention relates to a process for removing lubricant, hydrocarbons and mixtures thereof from a lubricated vapor compression mechanical refrigeration system by using CO.sub.2 as a solvent.
2. Description of Related Art
Recent legislation calls for the steady decrease in production of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) refrigerants. By 1996, there is projected to be no more production of CFCs, and by approximately 2010, there is projected to be no more production of HCFCs. The future production of these chemicals is being banned because they escape into the upper atmosphere and cause the decomposition of the protective ozone layer. As production of CFC and HCFC refrigerants is reduced and ultimately phased out, environmentally acceptable replacement compounds are needed for use in existing refrigeration systems.
The mechanical refrigeration industry has a very large number of units that are presently operating on CFC and HCFC refrigerants. The vast majority of these units use mineral oil lubricated motor driven compressors. These compressors are typically available in hermetic, semi-hermetic, and belt-drive configurations. The mineral oil is miscible (or dissolves) in the CFC and HCFC refrigerants, and is essentially similar to motor oil, except that the mineral oil is highly refined and specially dried for refrigeration service. It is preferred that refrigerants are substantially free of water for proper functioning in a mechanical refrigeration system.
The lubricants used in mechanical refrigeration systems cover a wide range of compositions. Those oils and lubricants that are commonly used in existing CFC and HCFC refrigeration systems include naphthenic, paraffinic, and alkyl benzene lubricants. The newer hydrofluorocarbon (HFC) type refrigerants require the use of polyalkyleneglycols (PAG) and synthetic esters which are commonly called polyolesters (POE). Also applicable for use with the HFC refrigerants are other lubricants such as silicones and polytetrafluoroethylene blends which are manufactured from other synthetic compounds.
In the normal operation of a mechanical refrigeration system, a small amount of the compressor lubricating oil travels with the refrigerant, through the evaporator coil, and eventually returns to the compressor. Miscibility, material compatibility and flowability of the oil/refrigerant mixture are important to guarantee that the oil does, in fact, return back to the compressor rather than stay in the evaporator, which would result in reduced refrigeration capacity.
The refrigeration industry has developed HFCs as one of the potential replacements for the CFC and HCFC refrigerants. HFC refrigerants offer improved environmental properties such as a zero ozone depletion potential and a significantly lower global warming potential, as compared with CFC and HCFC refrigerants. The new HFC refrigerants are designed to operate in pressure and temperature ranges similar to that of most of the existing CFC and HCFC refrigerants.
The HFC refrigerants differ from CFC and HCFC refrigerants in that the former require the use of synthetic lubricating oils. These synthetic oils are typically either the POE type or PAG type. The standard naphthenic and paraffinic mineral oil lubricants used with CFC and HCFC refrigerants are not miscible with the HFC refrigerants and such a mixture would cause oil return problems. Therefore, in the case of conversion to HFC refrigerants, the mineral oil must be replaced with a compatible lubricant to allow proper operation of the mechanical refrigeration system. Replacing the lubricant in a mechanical refrigeration system is a difficult and complicated process.
There are also refrigerant blends available that are primarily mixes of existing HCFCs and HFCs that will be manufactured for twenty to thirty more years. These have similar operating characteristics to the ozone depleting CFCs, but will eventually be phased out as the HCFCs are eliminated. The blends are typically semi-azeotropic, which means that they will tend to separate into their various components in the event that there is a slow leak. These refrigerants typically require the use of alkylbenzene lubricant, and can withstand a higher percentage of residual mineral oil to maintain proper operation. However, in systems with high percentages of lubricating oil external to the compressor, it still may be necessary to flush the original lubricant from the system.
The refrigeration industry is faced with the task of either replacing entire units with HFC designed and compatible systems, or converting existing ones to HFC refrigerants, which require synthetic lubricants. The mineral oil concentration in HFC systems is preferably reduced to below about 5% of the total oil concentration to prevent the residual mineral oils from accumulating in the evaporator, a problem commonly called "logging". Due to the poor miscibility of mineral oil in HFC refrigerants, a mixture of synthetic oil and mineral oil could cause disruption of the system operation. Allowable residual mineral oil levels are highly dependent upon system configuration and operating conditions. When the system shows signs of poor evaporator heat transfer or poor oil return to the compressor, it may be an indication that a further reduction in the residual mineral oil level is required. Thus, the refrigeration art seeks a suitable method to effectively switch CFC/HCFC to HFC refrigerants in such systems.
The manufacturer's standard recommended procedure for eliminating mineral oil from a refrigeration system being converted to HFC refrigerants is to use multiple oil changes. Typically, the CFC refrigerant is removed from the system and collected. The mineral oil is drained from the system and replaced with an equal amount of synthetic oil having a viscosity similar to that of the mineral oil. The original CFC refrigerant is reintroduced into the system, which is allowed to run for a number of days to mix and dilute the synthetic and mineral oil mixture. The contaminated synthetic oil is drained from the system and replaced with a fresh quantity of synthetic oil. This process is repeated one or more times, as necessary. Typically, the system is drained at least three times. After the final flush the CFC refrigerant is replaced with HFC refrigerant. The goal is to remove the original lubricant by the process of dilution with synthetic oil to a concentration of approximately 5% or less.
This procedure is capable of eventually removing the original lubricant, but requires multiple service calls on the part of the service technician, and is costly. In addition, the synthetic oils are quite expensive, and the large volume of contaminated synthetic oil generated by this process must be properly disposed of.
Another cleaning method includes the use of compressed gases to remove obstructions. Typically, this process includes applying pressure to one end of the system to blow out blockages in the system. The pressure builds up on the intake side of the obstruction until the obstruction breaks loose and is carried out of the system due to the velocity of the released gas pressure. For example, gaseous carbon dioxide and nitrogen have been used in the past by refrigeration technicians for removing obstructions, blowing out foreign materials and leak testing systems. These uses were limited to gaseous applications only, with the compressed gases used as nothing more than a pressurized media to mechanically clear tubing or detect leaks.
Another solution for eliminating conventional oils from a system being converted to HFC refrigerants is to use R-11 (tetrafluoromethane) or 1,1,1 trichlorethylene and other aggressive solvents. The R-11 is a CFC and has the highest ozone depletion potential, so its use is no longer a viable solution. Other solvents such as 1,1,1 trichlorethylene are ozone depleters, toxic, and present disposal problems. The cost of disposing of these contaminated solvents is considerable.
Thus, the preferred procedure presently known in the art for cleaning such systems is the multiple oil change procedure noted above. Consequently, the refrigeration art lacks an economical, quick, efficient and environmentally sound way to switch from CFC/HCFC refrigerants to HFC refrigerants in oil lubricated vapor compression refrigeration systems.
The art also lacks a simple method to remove lubricating oils and other contaminating hydrocarbons from refrigeration systems. A hermetic or semi-hermetic refrigeration compressor is designed so that the entire motor compressor assembly is inside a sealed pressure container to prevent refrigerant loss through shaft seals. The typical failure of such a system is a motor burnout. Since the motor is surrounded by lubricating oil and refrigerant, the resulting heat and electrical arc causes these components to breakdown into acids, ash and other hydrocarbons. These undesirable products are typically circulated throughout the entire system prior to failure.
A primary problem with motor burnouts is the inability to adequately clean the refrigeration system of all of the internally decomposed products. If not sufficiently cleaned out, residual acids, decomposed motor varnish, and contaminated refrigerant will dramatically reduce the life of a replacement compressor. Present methods in the art for solving these problems include flushing the system with clean refrigerant, multiple changes of the filter/dryer, as well as, adding additional strainers. Frequent replacement of filters and strainers is required to gradually purify the repaired system. Moreover, flushing with refrigerant is now difficult, if not impossible, due to laws preventing atmospheric discharge of refrigerants. Flushing systems with clean refrigerant contaminates the refrigerant with decomposed products making the contaminated refrigerant expensive to dispose of and not recyclable. In many cases, replacement compressors have failed prematurely due to inadequate removal of the contaminants from the remainder of the system after a burnout. Thus, the art is searching for methods to remove not only lubricating oils but other decomposed hydrocarbons from such refrigeration systems.